Matthew B. I agree with Tom A. On this issue. Many times different people use different measures of ROI, and each persons business model/education level is different which could account for some of the variance.
When talking about ROI, first you need to clarify a formula that will be used by all parties involved in the comparison.
This is how I consider return. (All calculations are based on annual returns)
Personally I shoot for 15% CoC (cash on cash or often referred to as cash flow) on my rentals. That is my net rental income (pretax) / my total cash invested (down payment, out of pocket closing repair and holding cost)
I look at ROI as net rental income (pretax) + appreciation / my total cash invested (down payment, out of pocket closing repair and holding cost)
I also look at throw off, which is net rental income (pretax) + appreciation + tax benefits from depreciation + principle reduction / my total cash invested (down payment, out of pocket closing repair and holding cost). Principle reduction is much more complicated due to the tax benifits part and is a topic for another post.
Anyways, the reason I went in to all of that is I have heard the term ROI used to describe all of these.